This invention relates to the generation of energy, and more particularly, this invention relates to a device for generating neutrons by a fusion plasma.
Nuclear fusion is one of the primary nuclear reactions. The name indicates an energy-releasing rearrangement collision which can occur between various isotopes of low atomic number.
There is a great deal of interest in fusion plasmas in the hope that they may be used to produce useful power. There are several advantages to a fusion reaction which make it so appealing. Since a primary fusion fuel, deuterium, occurs naturally and is obtainable in virtually inexhaustible supply (by separation of heavy hydrogen from water, one atom of deuterium occurring per 6500 atoms of hydrogen), solution of the fusion power problem can permanently solve the problem of energy production for mankind with far less pollution of his environment. As a power source, the small amount of radioactive waste products from the fusion reaction is another argument in its favor as opposed to fusion of uranium. Also, a fusion reactor, by virtue of the small amount of fusionable material in the reactor at any time, would not explode.
In a nuclear fusion reaction the close encounter of two energy rich nuclei results in a mutual rearrangement of their nucleons (protons and neutrons) to produce two or more reaction products, together with a release of energy. The energy usually appears in the form of kinetic energy of the reaction products, although when energetically allowed, part may be taken up as energy of an excited state of the product nucleus. In contrast to neutron-produced nuclear fission reactions, colliding nuclei, because they are positively charged, require a substantial initial relative kinetic energy to overcome their mutual electrostatic repulsion so that reaction can occur. The largest reaction cross-section for fusion is between a mixture of the heavy isotopes of hydrogen, deuterium and tritium, which is a hundred times larger than the next most probably fuel mixture, that of deuterium itself. Thus, the mixture of deuterium and tritium and deuterium alone are the primary fuels being considered initially.
Nuclear fusion reactions can be self sustaining if they are carried out at a very high temperature. That is to say, if the fusion fuel exists in the form of a very hot ionized gas of stripped nuclei and free electrons, called a "plasma", the agitation energy of the nuclei can overcome their mutual repulsion, causing reactions to occur. This is the mechanism of energy generation in the stars and in the fusion bomb. It is also the method attempted for the controlled generation of fusion energy. In this latter instance, the plasma is generated and confined by either electromagnetic fields or inertially. However, all experiments have failed to produce a self-sustaining reaction primarily because of the inability to confine the fusion reaction for a sufficient amount of time.
Previous nuclear fusion reactors for controlled, self sustaining nuclear fusion reaction have been built in order to establish the feasibility of generating useful power. These reactors, however, have not met with success, primarily because the amount of energy used to maintain the plasma has been greater than the energy generated. The reaction in such reactors has ordinarily been carried out in a very hot but tenuous fuel gas mixture of hydrogen isotopes. To avoid immediate quenching of the reaction, it has been carried out in an evacuated chamber, with means provided to prevent the reacting fuel from coming in contact with the chamber walls. The use of magnetic fields has been the method for achieving this. All of these reactors have failed, primarily because of the breakup of the plasma. There are, however, nuclear fusion research reactors which produce energy in short bursts and emit fast neutrons.
Two types of confinement are presently being used; the first and older approach is generally referred to as magnetic confinement, while the second and newer approach is called dynamic confinement. Magnetic confinement takes advantage of the fact that at the elevated temperatures required for fusion to occur (order of 10.sup.8 degrees) the atoms are stripped of their electrons (i.e., they are ionized) and are strongly affected and can be controlled by magnetic fields. Dynamic confinement relies upon the short times required (order of 10.sup.-9 seconds) for a high density solid (10.sup.23 atoms/cc) to meet the Lawson criteria of n.tau..apprxeq.10.sup.14 sec/cc for net energy production. Briefly, in one method, a short burst of a very high energy density flux is focused upon, and completely around, a small solid pellet of fusion fuel with the aid of split beams from an appropriate laser. The outer surface of the pellet is very quickly vaporized and almost explosively pushes itself away from the pellet. The pressure on the remaining solid increases sufficiently to increase its density to perhaps 10.sup.3 -10.sup.5 g/cm.sup.3. The resulting implosion is sufficient to initiate and sustain the fusion reaction and produce energetic neutrons.